This invention relates generally to medical imaging systems, and more particularly, to image reconstruction using Positron Emission Tomography (PET).
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid, glucose metabolism, and protein synthesis. As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation is characterized by two features which are pertinent to PET scanners—each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilations at each location within the field of view.
At least one known PET scanner is cylindrical and includes a detector ring assembly composed of rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected approximately simultaneously by detectors located on opposite sides of the patient. The number of such simultaneous events, i.e. coincidence events, indicates the number of positron annihilations that occur along a line joining the two opposing detectors. During an acquisition, coincidence events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the detector ring. These numbers are employed to reconstruct an image using well-known computed tomography techniques.
In order to accurately determine coincidence events and thereby obtain useful information for producing images, PET scanners utilize timing circuits to accurately identify and log the times at which photons are received at the detectors of the scanners. These timing circuits, which are often referred to as event locator circuits, typically include digital counters that count time periods based upon a digital clock, and digital counter latches that receive both the count signals from the counters and impulse signals from the detectors of the PET scanner whenever photons are detected. Based upon the count signals, the counter latches effectively time-stamp the impulse signals with times indicative of when the impulse signals are received, and output this information for use by the PET scanner in determining coincidence events.
More specifically, at least one PET scanner includes a digital timing circuit to perform Time-to-Digital Conversion (TDC) at a resolution of 1.302-nS. This known timing circuit includes a quadrature clock in which the frequency of the quadrature clock is fixed at 192-MHz and the quantity of phases of the TDC clock is fixed at four phases. During operation, the digital input to the TDC is driven by an analog comparator and may include very short pulses caused by noise in the comparator input.
To facilitate rejecting the short pulses caused by noise in the comparator input, the known digital timing circuit includes a circuit configured to reject pulses that were deemed to be too short in duration, i.e. less than 20 nanoseconds (nS) in duration. The timing circuit utilizes a 40-MHz clock, with a 25 nS period, through a 5-tap analog delay line, wherein each tap is set to 2.5 nS. Additionally, the timing circuit includes a counter running at 40-MHz to count the number of cycles that the pulse is active to determine the pulse width. However, a timing circuit that has a fixed resolution is may not be easily adapted to generate a desired resolution.